Generally pneumatic percussion machines comprise a hollow steel casing or cylinder, one end of which is closed off with a valve assembly through which compressed air is supplied to the piston face while the other end is closed off with a bit assembly.
Various arrangements have been proposed for bringing about reciprocation of the piston in the casing. An object of the present invention is the provision of a pneumatic percussion machine which it is believed will have advantages over many known arrangements.
According to the invention, there is provided a pneumatic percussion machine including:
A hollow casing; PA1 A valve assembly at one end of the casing; PA1 A bit assembly at the other end of the casing; PA1 A piston located in the casing and having an axially disposed bore extending the length of the piston; PA1 A projecting member extending from the valve assembly into the bore of the piston and along which member the piston is adapted to reciprocate; PA1 A first chamber formed between an end of the piston and the valve assembly; PA1 A second chamber formed at or towards the other end of the piston between the piston and the casing; PA1 A first fluid supply path through the valve assembly to the first chamber; PA1 A second fluid supply path extending for portion of its length through the valve assembly and projecting member and, for the remaining portion of its length through the piston to the second chamber, such portions of the second fluid supply path being brought into communication with each other over a limited range of positions of the piston along the projecting member; and PA1 A fluid exhaust path from the first chamber through the projecting member into the bore of the piston and then out through the casing end fitted with the bit assembly, such fluid exhaust path being open over a limited range of positions of the piston along the projecting member; the piston being adapted to move, as the valve assembly opens alternatively the first and second fluid supply paths, between a first position in which: PA1 It is in contact with the bit assembly; PA1 The second fluid path is open to the second chamber; and PA1 The fluid exhaust path is open; and a second position, displaced towards the valve assembly from the first position, in which the first fluid supply path is open to the first chamber.
The piston may be adapted to move to a third position, displaced towards the bit assembly from the piston's first position in which the portion of the second fluid path contained in the valve assembly and the projecting member is open to the first chamber and in which the fluid exhaust path is open. A bit forming part of the bit assembly may be adapted to move between a first position in which it is in contact with the piston in the latter's first position and a second position, displaced from its first position towards the casing end containing such bit assembly, in which the piston is in its third position.
Further according to the invention, a valve assembly includes a clapper valve adapted to open, during operation of the machine, the first fluid supply path during at least part of the movement of the piston from its second position to its first position and to open the second fluid path for at least part of the movement of the piston from its first position to its second position.
Also according to the invention, at least one of the fluid supply paths is open in the piston's third position.